The per-bag charge is assessed for each item that can't be stowed under the seat. The policy is in effect for reservations that were made after Monday for travel on or after Aug. 1.

Spirit Airlines Inc., the low-cost carrier based in Miramar, Fla., announced Tuesday that it would charge passengers as much as $45 for each piece of carry-on luggage.

The move is a departure from other airlines that have added or increased fees to check luggage into the cargo bay. Most airlines have added such fees in the last two years to help make up for declining demand.

Airline watchdog groups wonder whether the move by Spirit will prompt other airlines to do the same.

"The real question is will other airlines follow, and will this actually be good for air travel?" said George Hobica, president of Airfarewatchdog.com, an air fare comparison, alert and listing site.

Spirit, which flies to the Caribbean, Bahamas, and Central and Latin America, offers a daily flight from Los Angeles International Airport to Florida. The airline introduced the new fee while announcing new lower fares and bag fees for its $9 Fare Club members.

Carry-ons that fit in the overhead bin will cost fliers $45 -- $30 if paid online. Fare club members pay $20 for each carry-on bag.

There is no charge for carry-on items that fit under the seat.

The carry-on bag fee is in effect for reservations made after Monday for travel on or after Aug. 1.